The following U.S. Patents are believed to represent the current state of the art:
U.S. Pat. No. 7,278,580; U.S. Pat. No. 7,271,039; U.S. Pat. No. 7,269,021; U.S. Pat. No. 7,243,840; U.S. Pat. No. 7,240,847 and U.S. Pat. No. 7,204,427.